It is often important in the diagnosis and treatment of hearing problems to be able to determine with accuracy the acoustic pressure within the ear canal of a patient or the change in acoustic pressure introduced by a hearing aid. For example, the most accurate analyses of the hearing of a patient can be made by determining the actual sound pressure at the ear drum and relating these physical measurements to a patient's behavorial response. To obtain such physical measurements, it is necessary to provide a relatively nonintrusive means of measuring the pressure at or near the ear drum in a manner that does not substantially affect the transmission of sound through the ear canal. Usually the foregoing measurements are made by inserting a thin probe tube into the ear canal which transmits the sound pressure within the canal at the tip of the tube to an externally located microphone. Positioning of a probe into the ear canal must be made with great care to avoid contact with the ear drum. Precise and repeatable positioning is complicated by patient head and body motions and lack of cooperation in small children.
The measured value of a sound waveform in an open or closed ear canal is highly dependent upon the position of the tip of the sound collection tube, particularly relative to the frequency of the acoustic signal. Therefore, the accuracy and reliability of canal sound pressure level measurements are directly related to the stability and accuracy of the probe tube tip position. A second factor which can influence the absolute value of the measured canal frequency gain is the method by which the pressure is measured. Typically, a comparison is made between a microphone placed external to the canal and a microphone placed within the canal. This two microphone technique results in a differential recording which is very sensitive to placement of the reference microphone. Unfortunately, there is no consensus in the scientific or clinical community as to the "optimal" placement of the reference microphone. Conventional probe tube measurement instruments typically restrict the placement of the reference microphone to a specific location which may not be the location preferred by the user.